As of the end of 2007, an estimated 33 million people worldwide were living with HIV/AIDS, and the Centers for Disease Control and Prevention (CDC) estimates that 1,200,000 U.S. residents are living with HIV infection (UNAIDS/WHO AIDS epidemic update, December 2008; The Henry J. Kaiser Family Foundation HIV/AIDS Policy Fact Sheet, July 2007). Although new infections have decreased in recent years, an estimated 2.6 million new HIV infections occurred worldwide during 2007 and approximately 40,000 new HIV infections occur each year in the United States.
HIV entry within the target cells involves a series of molecular events. The three main steps of virus entry within the cell are: (i) attachment of the virus to the subject cells; (ii) interaction of the virus with the co-receptors; (iii) fusion of the virus and subject cell membranes. Considering the complexity of the molecular events involved in viral infection, all three of these steps have been considered for the drug design. The T-lymphocyte cell surface protein CD4 is the primary receptor involved in the interaction with the viral glycoprotein gp120, but a cellular co-receptor is also needed for the successful entry of the virus within the cell. At least two types of such co-receptors have been identified so far, both of which are chemokine receptors, CCR5 and CXCR4. These chemokine receptors are therefore gateways for HIV entry, determinants of viral tropism and sensitivity.
Compounds targeting viral entry have two advantages over those that target the HIV-1 reverse transcriptase or protease enzymes: entry inhibitors do not depend on efficient cellular uptake or intracellular activation processes to exert their biological effects, and they are highly unlikely to show any cross-resistance with protease inhibitors or reverse transcriptase inhibitors. Viral entry has been validated as a clinically effective pathway for targeted intervention by the first fusion inhibitor, enfuvirtide. Other classes of entry inhibitor under development target the initial binding of viral gp120 to CD4 and the interaction of gp120 with cell surface chemokine receptors that serve as co-receptors for HIV entry (CCR5 or CXCR4). Westby et al., Journal of Virology, 2006, 80(10), 4909-4920.
Compounds targeting CXCR4 have been developed primarily for treatment of HIV because CXCR4 is a major co-receptor for T-tropic HIV infection. For example, U.S. Pat. No. 6,429,308 discloses an antisense oligonucleotide to CXCR4 to inhibit the expression of the CXCR4 protein for use as an anti-HIV agent. International patent application publication number WO 2001/56591 describes peptide fragments of viral macrophage inflammatory protein II which are described as selectively preventing CXCR4 signal transduction and co-receptor function in mediating entry of HIV-I. Additional molecular antagonists of chemokine CXCR4 receptor are disclosed in international patent application publication numbers WO 2009/121063 and WO 2006/020415.
Studies have shown that CXCR4 interactions also regulate the migration of metastatic cells. Hypoxia, a reduction in partial oxygen pressure, is a micro-environmental change that occurs in most solid tumors and is a major inducer of tumor angiogenesis and therapeutic resistance. Hypoxia increases CXCR4 levels (Staller, et al. (2003) Nature 425: 307-311). Microarray analysis on a sub-population of cells from a bone metastatic model with elevated metastatic activity showed that one of the genes increased in the metastatic phenotype was CXCR4. Furthermore, over-expression of CXCR4 in isolated cells significantly increased the metastatic activity (Kang, et al. (2003) Cancer Cell 3: 537-549). In samples collected from various breast cancer patients, Muller et al. (Muller, et al. (2001) Nature 410: 50-56) found that CXCR4 expression levels are higher in primary tumors relative to normal mammary gland or epithelial cells. These results suggest that the expression of CXCR4 on cancer cell surfaces may direct the cancer cells to sites that express high levels of SDF-I. Consistent with this hypothesis, SDF-I is highly expressed in the most common destinations of breast cancer metastasis including lymph nodes, lung, liver, and bone marrow. Moreover, CXCR4 antibody treatment has been shown to inhibit metastasis to regional lymph nodes when compared to control isotypes that all metastasized to lymph nodes and lungs (Muller, et al. (2001) Nature 410: 50-56).
In addition to regulating migration of cancer cells, CXCR4-SDF-1 interactions may regulate vascularization necessary for metastasis. Blocking either CXCR4/SDF-1 interaction or the major G-protein of CXCR4/SDF-1 signaling pathway (Ga1) inhibits VEGF-dependent neovascularization. These results indicate that SDF-1/CXCR4 controls VEGF signaling systems that are regulators of endothelial cell morphogenesis and angiogenesis. Numerous studies have shown that VEGF and MMPs actively contribute to cancer progression and metastasis. Thus, there is a need to identify CXCR4 antagonists for therapeutic applications in treating or preventing cancer.